Inside a van, and rattling towards the airport, Sergei Skripal was in high spirits. It was July 2010. Without explanation Skripal had been taken from a penal colony, where he had spent the previous five and a half years, and transported in handcuffs to Moscow. Now he was about to board a flight to Vienna. His ultimate destination: Britain.

Unlikely though it seemed, Skripal was about to be swapped in classic cold war fashion. On the tarmac at Vienna airport he was to be exchanged for 10 Russian “sleeper agents” caught by the FBI and on their way home to Moscow. Heading in the other direction were three fellow Russians, including Igor Sutyagin. All were accused of working for UK or US intelligence.